This invention relates to a recording medium for ink jet recording and to the treatment of images prepared by an ink-jet printing process.
Ink jet printing is a non impact printing method that in response to a digital signal produces droplets of ink that are deposited on a substrate to produce an image. Ink jet printing has found broad application in industry as well as for output from personal computers in the home and office. There is increasing interest in the use of digital imaging with an ink jet printers as an alternative to conventional photographic imaging techniques. However the images produced by ink jet printers are seen as suffering several disadvantages when compared with conventional photographic images. In general they lack the overall quality of photographic images, look and feel substantially different, lack stability to light, and are more sensitive to water, scratching, rubbing, and environmental influences.
Aqueous inks are commonly used in ink jet printers for environmental and safety reasons, particularly those intended for use in the home or office. However sensitivity of the printed image to water is a particular problem where aqueous inks are used.
One way of overcoming these disadvantages is to laminate or encapsulate ink jet images, particularly those destined for external display. By lamination is meant the combination of a printed ink jet receiving layer with a transparent overlay, this combination usually being accomplished with an adhesive activated by heat, pressure, or both. The overlay acts as a physical protection for the image and completely seals it from ingress of water. By encapsulation is meant the combination of a printed ink jet receiving layer between two laminating sheets, that on the image surface being transparent, the combination being accomplished with an adhesive activated by heat, pressure, or both. Encapsulation is most effective if the laminating sheets extend beyond the ink jet image and are bonded to each other at the extremities, thus preventing ingress of water through exposed edges of the ink jet image.
However lamination and encapsulation both have disadvantages. They are expensive because additional materials are required together with additional handling and equipment. Moreover residual solvents such as the organic cosolvents which are frequently incorporated with aqueous inks remain trapped with the printed image, and these can sometimes degrade image quality by causing stain or migration of the image on storage or exposure. In addition the material of the laminate or adhesive can also deteriorate and cause stain on exposure. Laminates do not always adhere well to printed ink jet images, and adhesion can depend on the coatings of the ink receiving layer, the amount and type of solvent in the ink, and also on the quantity of ink laid down. This is particularly found when the ink jet image is being used instead of a conventional photographic image, as heavy ink loads are often used to reproduce the, image quality.
As an alternative to lamination, various additional coatings and treatments for ink jet receiving layers have been proposed. In most cases these are coatings such as lacquers or varnishes which have to be applied after printing the image, thus also requiring additional equipment. For instance British Patent 2 337 482 A provides a method for increasing the rub resistance of an ink jet image by coating or over-printing the image with an aqueous solution of a styrene acrylate polymer.
Various types of ink jet receiving materials are also known wherein the top layer or an upper layer of the material comprises a film forming polymer and the lower layer or layers comprise ink receiving layers, such that when the image is printed the ink passes through the upper layer or layers and is held by the lower layers. The material is subsequently heated above the film forming temperature of the polymer, which thus fuses to form a barrier layer which seals the image. Such heat sealing systems are disclosed for instance in Japanese Patent Applications 59/222381, 07/237348, 08/02090, and 09/104164 and in European Patent Applications 0 858 905 and 0 858 906. This method is limited, however, as a high temperature is necessary to melt the polymer (170xc2x0 C. in the Examples of EP 0 858 906), and special equipment is required to achieve this. Moreover not all substrates and ink receiving layers can withstand the high fusing temperature, and this restricts the generality of these methods. In addition the resultant image retains the solvents and can be subject to deterioration in the same fashion as a laminated or encapsulated image.
There is thus still a need for a convenient and general method for protecting ink jet images. We have found such a method.
According to the present invention there is provided an ink jet printing method which comprises the steps of:
1) printing on to a receiving medium which comprises a substrate coated with at least one ink receiving layer and at least one upper protective layer which comprises polymeric particles having film forming temperatures between 60 to 140xc2x0 C., preferably between 100 to 120xc2x0 C., and at least one binder, and
2) subsequently heating the printed image to form a stable image protecting coating.
In contrast to the materials previously known in the art, the image in the materials of this invention is substantially retained within the upper protective layer. It is believed, however, that any retained solvents are held in the lower image receiving layers, thus separating them from the colorant.
The protective layers of the invention are receptive to inks during printing, give high quality images of good colour strength, adhere well after printing and fusing, provide a high level of scratch and rub resistance to the final image even when wet, and maintain the same level of flexibility as the rest of the assembly.